1. Field of the Invention
The present invention relates to an optical interconnection devices for transmitting signals within a data-processing apparatus and the like, and more particularly to an optical interconnection devices for transmitting optical signals through space.
2. Description of the Related Art
The operating efficiency and speed of a dataprocessing apparatus and the like have increased. In other words, the speeds at which the signals are processed and transmitted in the apparatus have increased. The speed increases are mainly due to the high signal-processing speed and the high signal-transmitting speed are attributed to the improved performance of the semiconductor IC devices incorporated in the apparatus. Also, they are greatly attributed to the advanced integration technique applied to the semiconductor IC devices and the advanced technique of mounting the IC devices.
The performance and integration density of semiconductor IC devices can be increased by virtue of the foreseeable improvement in the structural design and manufacture process. It is therefore well expected that a data-processing apparatus or the like will have an even higher operating speed. However, the operating speed of the apparatus may be limited despite the increased performance and integration density of semiconductor IC devices, unless the semiconductor IC devices are connected by optical interconnection devices.
Electrical connection among the elements of each semiconductor device can be easily achieved without affecting the operating speed of the data-processing apparatus or the like, whereas electrical connection among the semiconductor IC devices can hardly be accomplished without detriment to the operating speed of the apparatus. The reason will be described in the following.
In each semiconductor IC device, the lines interconnecting the semiconductor elements are tens of micrometers long at most. The free-space equivalent connection length is only a few times as long as the actual interconnection length. The electrical connection length is far shorter than the wavelengths of signals (e.g., about 30 cm for 1 GHz signals). Reflection of signals, if any, caused by impedance mismatching due to wave propagation effect is almost negligible. Also, since the substrate of the semiconductor IC device, in which elements are integrally formed, has a relatively great dielectric constant, usually 10 or more, the electric and magnetic fields are likely to concentrate near the substrate. The electromagnetic coupling (i.e., crosstalk) between any adjacent interconnection line is weaker than that in free space. Hence, the semiconductor IC device can operate at a relatively high speed.
The electrical connection among the semiconductor IC devices involves a delay in transmitting signals and frequency-dependence of transmission loss, which can be attributed to the parasitic resistances and parasitic capacitances of the interconnection lines, and causes a crosstalk due to the electromagnetic coupling among the interconnection lines connecting the ICs. The ability of each IC device is reduced in effect by the external factors, i.e., the delay in transmitting signals, the frequency-dependency of transmission loss, and the cross talk.
The crosstalk caused by the electrical connection among the semiconductor IC devices is much greater than one occurring in each IC device. This is mainly because the interconnection lines of the IC devices are far longer than the interconnection used in each IC device. Even in a multi-chip module which is a hybrid module comprising several IC chips connected together, the electric connection length is as much as about 1 cm. In some multi-chip module, the electric connection length is as much as tens of centimeters. In either case, the impedance mismatching due to wave propagation effect is too prominent to neglect. In order to minimize the impedance mismatching it is necessary to set each interconnection line at a specific impedance such as transmission line, making it difficult to set the width of the line at any desired value. Consequently, each interconnection line cannot be narrowed to reduce its parasitic capacitance. Ultimately, the transmission of signals will be delayed greatly.
As a consequence, even if the systems comprises electrically connected semiconductor constituent IC devices which operate at a clock frequency of hundreds of megahertz or more, the systems can operate but at such a low speed as if it comprised IC devices which operate a clock frequency of tens of megahertz. To state in other words, the operation speed of the systems cannot increase in proportion to the speed of its constituent IC devices electrically connected together.
In order to solve this problem, various methods of interconnecting semiconductor IC devices by optical interconnection devices have been proposed. In an apparatus comprising IC devices connected by optical interconnections, there occurs neither a signal transmission delay nor a crosstalk. This is because no electrical lines are used to connect the high speed signal IC devices, and there are no problems resulting from the parasitic resistances or parasitic capacitances of such electrical lines, or from the electromagnetic coupling among such electrical lines interconnecting the IC devices.
Optical interconnections will provide a advantage when used in a data-processing systems or the like, optically interconnecting the boards which hold semiconductor IC devices and which are spaced from one another by a relatively long distance. If the distance is as long as several meters, the transmission band width and the transmission loss are as wide and small as those occurring in the case where the boards are spaced apart by a distance of about 1 cm and electrically connected. Furthermore, a signal transmission delay, if any, is far smaller than the delay which inevitably takes place in the case where the boards are electrically connected.
A method of interconnecting IC devices is proposed, in which electrical lines are used to connect the IC devices mounted on each board and optical interconnections device are used to optically connect adjacent boards. The optical interconnection devices will be described, with reference to FIGS. 1A and 1B, FIGS. 2A and 2B and FIGS. 3A and 3B.
FIG. 1 is a schematic representation of the input/output section 9 of a conventional optical interconnection device. As shown in FIG. 1, the section 9 comprises a light-receiving element 1, an output terminal 2 connected to the element 1, a light-emitting element 4, an input terminal 5 connected to the element 4, a collimating lens 7 located in front of the light-receiving surface of the element 1, and a collimating lens 8 located in front of the light-emitting surface of the element 4. The light-receiving element 1 has a light-receiving region 3 formed in the light-receiving surface. The light-emitting element 4 has a light-emitting region 6 formed in the light-emitting surface.
As shown in FIG. 1B, the input/output sections 9a to 9e of the optical interconnection system of the type shown in FIG. 1A, which are mounted on IC-boards, respectively, are positioned such that the optical axes of the sections 9a to 9e are aligned with one another. The boards are thereby optically connected. In FIG. 1B, the arrows indicate the directions in which signals are transmitted. The input/output section 9a of the leftmost optical interconnection device may be connected by an optical fiber to the input/output section 9e of the rightmost optical interconnection device, to thereby form a loop bus.
The optical interconnection system shown in FIG. 1A is one of the simplest. Nonetheless, it can transmit signals between two adjacent boards, just in the same way as if the substrates were electrically connected. Connected by the optical interconnection device, the boards can be separated by a relatively long distance. In addition, the transmission band width for each optical path can be as wide as 10 Gb/s. A data-processing apparatus or the like will, therefore, have its operating speed much increased if its constituent IC devices are optically connected by the interconnections of the type shown in FIG. 1A.
The optical interconnection device of FIG. 1 serves to broaden the transmission band width, but not to increase the transmission speed to a value much greater than is achieved by the electrical interconnecting lines. The reason is that, although signals are transmitted fast between the input/output sections 9 of any adjacent optical interconnection devices, it takes some time to process the signals supplied to the board and to transfer the signals among the IC devices via the lines interconnecting these IC devices. Consequently, in the apparatus as a whole, the signal transmission is delayed by the time required to process the signals in all IC devices incorporated in the apparatus. This delay time is as long as a period of several clock pulses even in the case where a data buffer and a bypass are mounted on each mounting board, or as long as several nanoseconds even in so-called simple bypass method in which the signals are analog-processed.
Hence, there will be no advantage in using the optical interconnection devices of the type shown in FIG. 1A unless the signals supplied from each substrate are processed by the IC devices mounted on the immediately next substrate. For example, if the signals from the input/output section 9a are transmitted to the input/output section 9e via the intervening sections 9b to 9d and subsequently processed by the IC devices optically connected to the section 9e, they will be delayed. Similarly, the signals will be delayed if they are transferred back to their source IC device after they have been processed by another IC device. Furthermore, the signals are delayed by a period while processed on one board and by a different period while processed on a different substrate, resulting in non-uniform signal transmission among the IC-boards.
FIG. 2A shows an optical interconnection system disclosed in Published Unexamined Japanese Patent Application No. 5-119275, which is an improvement of the optical interconnection device illustrated in FIG. 1A. This system is a little more complicated than the interconnection device of FIG. 1A, but solves the problems inherent in the optical interconnection system of FIG. 1A.
As shown in FIG. 2A, the optical interconnection system comprises a surface emitting semiconductor laser 11 (i.e., a light-emitting element), an input terminal 12 connected to the laser 11, semitransparent light-receiving elements 14a, 14b . . . , output terminals 16a and 16b connected to the elements 14a and 14b, respectively, a non-transparent light-receiving element 17, and an output terminal 18 connected to the element 17. The semiconductor laser 11 has an active region 13 from which to emit a laser beam. The light-receiving elements 14a and 14b have light-receiving regions 15a and 15b, respectively. The light-receiving element 17 has a light-receiving region 19 and functions as the last component of the optical path it defines jointly with the other components 11, 14a, and 14b.
The optical interconnection system further comprises collimating lens 20 and 21 and beam-correcting lenses 22a, 22b, . . . The collimating lens 20 is located between the surface emitting semiconductor laser 11 and the light-receiving element 14a. The first beam-correcting lens 22a is located between the light-receiving elements 14a and 14b, the second beam-correcting lens 22b is located between the second light-receiving element 14b and the third light-receiving element (not shown), and so forth. The last beam-correcting lens (not shown) is located between the penultimate light-receiving element (not shown) and the last light-receiving element (not shown). The collimating lens 21 is located between the last light-receiving element and the non-transparent light-receiving element 17. The lenses 22a, 22b, . . . converge the laser beams which have been diverged by diffraction and applied to them, so that laser beams of the same diameter may be applied onto the light-receiving elements 14a, 14b, . . .
Generally, the diameter .omega. of a beam can be represented as follows: EQU .omega..sup.2 =.omega..sub.0.sup.2 {1+(.lambda.Z/.pi..omega..sub.0.sup.2).sup.2 } (1)
where
.omega..sub.0 : the minimum diameter of the beam PA1 .lambda.: the wavelength of the light beam PA1 Z: the distance from the waist of the beam
The minimum beam diameter .omega..sub.0 and the beam diameter have the relationship illustrated in FIG. 2B in the case where the wavelength .lambda. is 1 .mu.m and the distance Z is about 25 mm (about 1 inch), i.e., a general value for the distance between the aboards which the interconnection device connect optically.
Assume that the laser beam being emitted from the active region 13 of the surface semiconductor laser 11 has a diameter of 4 .mu.m. The moment the beam transmitted the distance Z of 25 mm, its diameter has increased to about 2000 .mu.m, or about 500 times greater than the initial value (4 .mu.m). In order to prevent this excessive diversion of the laser beam, the collimating lens 20 converts the input laser beam to substantially parallel beam having a diameter of, for example, 100 .mu.m. The moment the beam transmitted another distance Z of 25 mm from the lens 20, its diameter has increased to about 128 .mu.m. Hence, if the beam further transmitted a distance ten times longer (250 mm), its diameter will increase to about 800 .mu.m. To avoid this increase in the beam diameter, the beam-correcting lens 22a, 22b, . . . are arranged on the output sides of the light-receiving elements 14a, 14b, . . . , respectively. These lenses 22a, 22b, . . . converge the input beam, to thereby maintain the diameter of the laser beam at a constant value.
The optical interconnection system shown in FIG. 2A is characterized in that a signal supplied to the input terminal 12 can be transmitted from one board to another, virtually at the same time, without involving in any electrical processing and, hence, without being excessively delayed. In other words, this interconnection system can reduce the transmission delay to a minimum and can transmit a signal to all boards simultaneously. To transmit signals in the opposite direction, another similar optical interconnection system defining an optical path is used, which has the same advantages as the interconnection device shown in FIG. 2A.
To transmit optical signals backwards, a plurality of identical optical interconnecting paths of the type shown in FIG. 2A are juxtaposed in the same number as the boards, with their semiconductor lasers 11 arranged in staggered fashion. The optical interconnecting paths may be arranged in a circle, thus forming a loop path. Alternatively, they may be optically connected by reflectors, thereby forming a loop path. In either case, each optical interconnecting path for one board has one transmitting element and light-receiving elements the number of which is one less than that of all boards. Optical signals can be transmitted at high speed among the boards even if the substrates are at relatively long distance from from one another.
The optical interconnection system shown in FIG. 2A is disadvantageous in two respects, because of its complex optical system. First, the components must be positioned accurately in assembling the interconnection device. Second, its reliability may be affected by changes in the temperature.
To be more specific, the lenses 20, 21, 22a, and 22b must be set in axial alignment with high precision and, in addition, the light-receiving elements 14a, 14b, . . . must be located exactly at the beam waists. It is comparatively easy to place the lenses in axial alignment, but it is difficult to detect the beam waists and locate the light-receiving elements at these beam waists. To make matters worse, each light-receiving element must be adjusted for its degree of parallelism. When the element is adjusted for its degree of parallelism, its optical axis may come out of alignment with those of the adjacent elements. Even if the optical axis is deviated only a little from that of the next element, the optical signals may not be transmitted to the next element. Further, the optical axis of each element and that of the associated lens may deviate from each other when the temperature changes. This is because the elements are made of one material, and the lenses are made of another material.
As has been described, the conventional optical interconnection devices are disadvantageous in that signals are delayed while electrically processed within the interconnection devices, or in that their optical systems are complex and difficult to assemble and that may fail to operate reliably when the temperature changes.